Frequency spectrum analyzers are widely used for analyzing frequency spectrum of an incoming signal in a frequency domain. Typically in such a frequency spectrum analyzer, levels of frequency spectrum are displayed in a vertical direction with respect to a frequency range in a horizontal direction. A frequency spectrum analyzer may also include a function for displaying levels of the incoming signal in a time domain. A conventional example of such a frequency spectrum analyzer having frequency domain and time domain analysis capabilities is shown in FIG. 6.
The conventional frequency spectrum analyzer of FIG. 6 includes an RF section 10, a detector 20, an AD converter 30, a microprocessor 50, a display 60, and a frequency reference source 70. The RF section 10 is formed of an attenuator 11, an amplifier 12, a frequency mixer 13, a local frequency oscillator 15 and an intermediate frequency (IF) filter 14.
When the frequency spectrum analyzer of FIG. 6 is used for analyzing frequency spectrum of an incoming signal in the frequency domain, the local frequency oscillator 15 linearly sweeps its frequency (sweep mode) for a specified frequency range. When the frequency spectrum analyzer is used for analyzing a time domain waveform of the incoming signal, the local frequency oscillator 15 is set a fixed frequency (zero span mode).
First, the basic operation of the frequency domain analysis is described in the following: An input RF signal provided to an RF terminal is adjusted its power level by the attenuator 11 and the amplifier 12 before being applied to the frequency mixer 13 in such a way that the maximum dynamic range is attained in the measured results. Typically, such a measurement dynamic range is determined by the maximum possible input power level to be applied to the frequency mixer 13 without distortion.
In the example of FIG. 6, the local frequency oscillator 15 generates a local signal whose frequency is swept linearly (sweep mode) with reference to the reference frequency source 70. The RF signal frequency and the local signal frequency are mixed in the frequency mixer 13, thereby creating IF signals having both sum and difference frequencies between the two frequencies. The IF filter 14, which is a band pass filter, selects either one of the sum or difference signals from the frequency mixer 13.
The detector 20 detects an amplitude of the IF signal from the output of the IF filter 14. The AD converter 30 converts the amplitude of the IF signal to a digital signal. The resultant digital signal generated by the AD converter 30 is processed by the microprocessor 50 and is displayed on the display 60 as frequency spectrum with power levels. Typically, the vertical axis of the display 60 represents power levels of the spectrum while the horizontal axis represents frequencies of the spectrum. The microprocessor 50 also controls overall operation of the spectrum analyzer including that of the local oscillator 15, detector 20, display 60 and AD converter 30 via a system bus 80.
Second, the basic operation of the time domain analysis in the frequency spectrum analyzer is described in the following: An input RF signal is provided to the frequency mixer 13 through the attenuator and amplifier in the same manner as in the frequency domain analysis. However, the local frequency oscillator 15 is tuned and fixed to an appropriate frequency (zero span mode) so that an IF signal which has a frequency equal to the center frequency of the band pass filter 14 is produced all the time by the frequency mixer 13. The IF signal from the filter 14 is amplitude detected by the detector 20 and converted to a digital signal by the AD converter 30. Therefore, on the display 60, the power level of the IF signal, which is proportional to the input RF signal, is shown in the time domain.
Thus, in the conventional frequency spectrum analyzer, in the time domain analysis, it is possible to observe and analyze the changes in the power levels of the input RF signal with respect to the elapse of time. However, it is not possible to observe and analyze the changes in the frequency or time period of the input RF signal with respect to the elapse of time. This is because the input RF signal is displayed on the screen in the same manner as displayed by an oscilloscope where a waveform is shown in a format of power level versus time.
Thus, in the zero span mode of the conventional frequency spectrum analyzer, input RF signals whose time period or frequency changes quickly cannot be properly measured in the time domain. For example, settling times in a VCO (voltage controlled oscillator) or PLL (phase lock loop) circuit or a frequency deviation in an FM (frequency modulation) system is not effectively measured by the conventional spectrum analyzer.